In recent years, mobile ad-hoc networks (MANET) have attracted attention as communication means for use at the time of disasters or in remote areas where network infrastructure has not yet been deployed. As in the case of network infrastructure, there is a demand for implementing mobile ad-hoc networks in a large-scale environment in which a wide range and a large number of communication terminals participate.
In the mobile ad-hoc networks MANET, however, communication terminals directly communicate with each other using wireless communication. Accordingly, the communication radio waves of the communication terminals participating in the network interfere with each other, resulting in limitation of available communication bands. In addition, communication links among the communication terminals can be frequently disconnected, or the communication terminals can be disconnected from the network due to movement of the communication terminals or interruption of communication radio waves by obstacles.
Since a network is constructed of a number of communication terminals, a huge amount of information on routes to a destination communication terminal is necessary for establishing a communication, which results in further narrowing the bands available to each communication terminal.
In the environment in which the available communication bands are narrow and the network can be disconnected, the following conditions are necessary for means to recognize a communication route.
(Condition 1) Scalability for preventing an increase in route information, and
(Condition 2) Robustness of route recognition to tolerate disconnection of a network.
Regarding the former Condition 1, as a mobile ad-hoc network MANET technique, a method is proposed in which the amount of route information is further reduced by constructing a network with a hierarchical structure as disclosed in “Scalable Routing Protocols for Mobile Ad Hoc Networks” by Xiaoyan Hong et al. (Non Patent Literature 1) and “Safari: A self-organizing, hierarchical architecture for scalable ad hoc networking” by Shu Du et al. (Non Patent Literature 2), and another method is proposed in which the amount of route information is further reduced by dynamically assigning addresses used for routing control as disclosed in Non Patent Literature 2.
However, Non Patent Literatures 1 and 2 do not propose any methods that satisfy the latter Condition 2 as well. This is because in the mobile ad hoc networks MANET, communication terminals can constantly and stably communicate with each other via a network connection, and a range in which a route to a destination can be determined by a routing protocol is defined as a routing domain. Therefore, if the network is disconnected, the route to the destination is unknown until each of disconnected routing domains is connected, even when it is attempted to transmit data to a communication terminal belonging to another routing domain. This makes it difficult to communicate with the communication terminal.
On the other hand, as a method that satisfies Condition 2, Delay/Disruption Tolerant Network (DTN) routing technique is proposed as disclosed in “Routing Method and Node” of Japanese Unexamined Patent Application Publication No. 2008-205890 (Patent Literature 1) and “Internet Draft draft-irtf-dtnrg-arch-04.txt” by the DTN Research Group (Non Patent Literature 3). The DTN routing technique has a feature that data to be transferred to a destination is accumulated. The technique involves an operation in which communication terminals that have encountered each other exchange route information on the other communication terminal and deliver the accumulated transfer data to a communication terminal which is most likely to be able to deliver the data to the destination. This enables communication with other desired communication terminals even when the network is disconnected and a plurality of routing domains are present.
However, since a disconnection of a network is assumed from the beginning, the DTN routing technique disclosed in Non Patent Literature 3 employs a mechanism for exchanging route information between the communication terminals that have encountered each other, regardless of routing domain. For this reason, in Non Patent Literature 1 described above, the amount of route information cannot be reduced, unlike in in Non Patent Literature 2, and the route information is held in each communication terminal belonging to the network. As a result, it is impossible to satisfy both of Condition 1 and Condition 2.
In this regard, both of the Conditions 1 and 2 can be satisfied by combining the MANET technique and DTN routing which solve the problems inherent in the route information described above. However, the combination of these two techniques is not sufficient to determine whether to exchange information by reducing the amount of route information based on the MANET technique between adjacent communication terminals, whether to exchange the route information based on DTN routing, and whether the MANET technique or the DTN technique should be executed. Accordingly, the communication terminals that have encountered each other constantly exchange the route information. This makes it difficult to satisfy both of the Conditions 1 and 2.
Under such circumstances, there is a strong demand for implementing a method capable of identifying a plurality of routing domains, determining whether the MANET technique or the DTN routing technique should be executed, and controlling route information to be exchanged in an environment in which a number of communication terminals participate in mobile ad-hoc networks MANET with a narrow band and the network can be disconnected.